Hydraulic control valves have been used in vehicle steering systems for many years and they have exhibited various configurations. One such control valve is taught in FIG. 4 of the French Pat. No. 1,465,422. Here a valve spool is shown having two lands which close off two working ports when the valve spool is in the neutral position. As the valve spool is displaced, one of the working ports is connected to a control pressure chamber while the other working port is connected to a tank port. The main pump in the hydraulic circuit is connected directly to both of the control pressure chambers by way of respective check valves and when the valve spool is in the neutral position, is connected to the tank port by way of a short-circuit path. Upon displacement of the manually operable steering wheel, the pressure in the two control pressure chambers is altered such that the valve spool is moved from its neutral position. Pressurized fluid then flows from the main pump into one of the piston-cylinder assemblies while the other pressure chamber is connected to the tank. In this arrangement, the control valve can be used only in a constant flow system because the control valve is open in the central position. Also, the control pump is in a pressure-less condition when the valve spool is in the neutral position. The disadvantage of this kind of control device is that there is no possibility of finely controlling movement of the valve spool. Therefore, the steering wheel must be continuously turned throughout the entire steering operation for otherwise the valve spool will move back to its neutral position. When this control valve is used in a road vehicle, there is the likelihood that the association between the steering wheel position and the position of the vehicle's wheels will change. Furthermore, the pressurized oil can only be coarsely metered by the control pump during rotation of the steering wheel.
A second type of hydraulic control valve is taught in German Pat. No. 2,946,274. In this patent, a control valve is described which can be used with a piston-cylinder assembly which incorporates a differential piston. A control valve uses two throttle means (29 and 30) which are adjustable in opposite directions to form a pressure divider. The disadvantage of this type of valve is that when an external force arises, the manually operable steering wheel must be held stationary or otherwise it will rotate in accordance with the movement of the piston. A second disadvantage is that the pressure within the valve must first be built up after the engine is started. Such a buildup in pressure is possible only if either the steering wheel or the vehicle's wheels are turned. This may at times be difficult, for example, when the vehicle's wheels are locked by a curbstone.
Now an improved hydraulic control valve has been invented which can be used in a constant pressure steering system on a vehicle and which permits fine control.